[The effect of melatonin and mexamine on the human thyroid under in-vitro conditions]

In in vitro study of the human euthyroid and thyrotoxic thyroid gland melatonin (N-acetyl-5-methoxytryptamine) and, to a lesser measure mexamine (5-methoxytryptamine) had a dose-dependent inhibiting effect on thyroxine secretion. Moreover, melatonin weakened the TSH stimulating effect in relation to the secretory process in the thymus while mexamine did not. Despite the similarity in the quality of the effect of both methoxyindoles on the release of thyroxine, the mechanism of its realization differs: the action of melatonin is mediated by the adenylate-cyclase-cAMP system, but in the action of mexamine the cAMP-dependent mechanism does not take part. Maintenance of the sensibility of the human thyroid to the effect of TSH is an obligatory condition for realization of the action of both methoxyindoles on the secretory process in it.